This invention relates to a method of measuring the rotor flux of a dynamoelectric machine by measuring the terminal voltage of the machine.
Many synchronous and induction motor control schemes such as vector control and sin .theta. controls use an estimate of the phase and amplitude of the rotor flux in determining motor torque and to generate frequency and current commands for the machine. In motor controls such as vector controls, the phase and magnitude of the flux must be accurately known since flux is produced by providing current in phase with the motor flux and torque is produced when flux and current are ninety degrees out of phase.
One method of measuring rotor flux is to use flux coils embedded in each phase of the motor stator as described in Franz et al, U.S. Pat. No. 4,011,489. However, this method is only available for motors specially constructed for flux control. It is desireable, therefore, to provide a method of flux control which can be applied to a standard alternating current motor.
If the voltage across the magnetizing reactance of a motor is integrated by an ideal integrator, the rotor flux is obtained. The voltage across the magnetizing reactance in a motor, however, cannot be measured directly. What can be measured is the total terminal voltage of the motor which can be represented as the sum of voltages due to stator resistance and leakage reactance in series with the induced CEMF (counterelectromotive force). The stator resistance cannot be easily compensated for since it varies as a function of temperature. Furthermore, the terminal voltage measurement is not exact due to minor voltage offsets in the voltage measuring circuits.
One approach for generating a flux signal from motor terminal voltage is to use a low pass filter as an integrator. At low frequencies, the low pass filter has a finite gain. The rotor flux is a sinusoidal varying signal which ideally has a long term average value of zero. If the voltage measuring circuit including the filter introduces any offset, the integrated signal will have a non-zero average value producing inaccurate results. When a low pass filter is used, precision voltage sensors with an accuracy greater than 0.1 percent are usually required.
Another approach to flux reconstruction, disclosed in Blaschke's German Pat. No. 1806769, is to use an integrator with a proportional plus integral feedback loop to generate a signal with a zero time average. The time constant of the proportional plus integral feedback has to be sufficiently large to provide averaging at low frequencies which in turn provides low gain and sluggish response. Distortion of the flux phase information occurs at low frequencies since the circuit is not an ideal integrator.
Yet another approach, disclosed in Dreiseitl et al, U.S. Pat. No. 4,282,473, is to use an adaptive notch filter to perform machine terminal voltage integration to avoid deterioration in performance at low frequencies. However, such technique introduces additional filter elements which may give rise to other offset problems. Furthermore, the motors are operated over a wide frequency range in which the notch filter is not utilized.
It is an object of the present invention to provide an improved method of obtaining flux information from motor terminal voltage.
It is an object of the present invention to generate a flux signal from the motor terminal voltage that provides DC elimination synchronous to the flux wave.
It is a further object of the present invention to generate a flux signal from terminal voltage that provides accurate phase information from low precision sensors.
It is a further object of the present invention to generate a flux signal from terminal voltage that provides DC elimination during rapidly changing rotor speeds.